Today, for matters of economics, there is more and more emphasis on increasing well production from a single well bore. Therefore, multi-zone well completions have become increasingly popular where geological strata permit. However, sand production even in single zone wells from unconsolidated formations continues to present problems in well production due to migration of loose sands into the wellbore as the formation deteriorates under the pressure and flow of fluids therethrough. The sand particles may clog the flow passages and can seriously erode the equipment. In some instances the clogging of the production system can lead to a complete cessation of oil flow. In short, every year the oil industry spends millions of dollars correcting problems caused by sand production and incurs reduced revenue from restricted production rates.
One method of limiting the damage of sand fines is by the introducing a gravel pack about the exterior surface of production screens placed in the well adjacent to the producing formation. The screens are installed during the completion stage of well drilling when the production equipment is put in place in the well bore. The screens are designed to prevent the flow of sand fines into the production tubing along with the produced fluids. Conventionally, the screens comprise one or more wrapped wire well screens that are subsequently packed with sand, gravel or epoxy coated gravel.
The traditional way of preventing fines from clogging the production screens is a process called packing-off. Packing-off involves isolating a production zone of a well and pumping a slurry of water, gravel, and sometimes a gel into the zone between the well casing and the production screens. The "gravel" is sand of a size chosen to be trapped on the outer surface of the screens. Once packed, the slurry or proppant may also be used under pressure to fracture the oil producing formation to effect more efficient production from the well. During packing, fluid pressure is used to cause the particulate part of the slurry to form a permeable mass around the screen and within the casing, sometimes extending into the production zone. The water is forced from the slurry through the screens and back up the well by the packing pressure, leaving the particulate behind. Because of the pressure, this permeable mass conforms to the shape of the interstitial void and forms a filter that will act on the well production. As a consequence, sand fines are trapped in the permeable mass, rather than entering the production tubing and causing problems in the production equipment.
The actual procedure of packing-off is begun with the assembly of a packer setting work string. The major parts of the work string are: a packer setting tool, a packer setting assembly, a packer and, in a multi-zone well, one or more of each of the following: isolation packers, closing sleeves, and production screens. In the case of multi-zone wells, this work string may be hundreds of feet long. The bottommost packer is usually a sump packer and performs the function of inhibiting fluid flow from within the casing into the geologic structure at the end of the well. Packers above the sump packer are termed isolation packers. They generally serve to create production zones between two isolation packers. In multi-zone wells, each zone is bounded above and below by a packer, which inhibits the packing fluid flow within the casing into adjacent producing or non-producing zones. Each of the packers is set in place by hydraulic pressure or mechanical motion that expands flexible elements against the inside surface of the casing. The elements are held in their compressed state by internal locking slips that prevent the elements from relaxing.
Because geological strata capable of producing oil and gas do not occur at regular vertical intervals, the location of the isolation packers must accommodate the locations of the production zones. Therefore, the packers are assembled to the work string with the location of the production zones in mind. Just above the uppermost production zone, a primary packer is inserted in the casing to isolate the fluid head above the production zones from the production zones. This fluid head creates an overburden of pressure above the packer assembly for well control purposes. However, it is undesirable for that fluid to be directly admitted into the producing formation because it can cause damage to the formation. Also, the chemicals in the fluid are expensive and they may be recovered if they are not flushed into the formation. This uppermost packer is equipped with bi-directional metal "slips" that are retracted during work string insertion, and are extended by hydraulic pressure exerted through a packer installation tool. When extended, the slips bite into the inner casing wall to securely hold the packer and production string in place against well pressures.
Once the packers are positioned and set in the well casing, the tool string is removed and a pack-off tool string is inserted into the well. Pack-off is accomplished by vertically positioning the pack-off assembly so as to introduce the slurry into each zone, usually from the lowermost zone progressively to the uppermost zone. Conventionally, this requires one tool trip per zone to gravel pack the zone and one to complete the zone. Gravel packing and completing requires two tool string trips per zone. Thus, a three-zone well would require six tool string trips a very time and labor intensive process.
During the packing-off process the fluid integrity of the zone being packed off must be assured, preventing fluid from flowing into adjacent producing or non-producing zones. While the packers seal against the inside of the casing, the seal must be completed between the tool string and the inner diameter of the packer. This is traditionally achieved by using o-ring seals on the outer diameter of the tool mandrel, sealing against the honed inner diameter of the packers that are fixed in the well casing. In a single zone completion this is not a major problem, as the distance between the packer and the screen is easily determined. In multi-zone completions, however, because of the irregular distances between production zones, sealing the zones becomes a problem. One skilled in the art will recognize that the location of the zones dictates the position of the packers. In order to accomplish the sealing as the work string is pulled progressively up the well to pack-off each zone, seals would have to be installed every one to two feet of mandrel length in order to align with the irregular location of the packers. This quickly becomes impractical when the tool string length is several hundred feet long or extremely expensive as the number of seals increases. Although the seals themselves are a significant cost, the machining costs of providing seal recesses at this spacing is significantly greater, thus increasing the cost of the tool string. Before the well is placed into production, the pack-off work string is retrieved to the surface, and replaced with a production string. The packers remain in the casing, to isolate the production zones.
Completion structures of the type disclosed in U.S. Pat. Nos. 5,332,045 and 5,180,016, which are incorporated herein by reference, are often used in gravel pack well bores. These completion structures are generally comprised of several different completion apparatus that are coupled together and work in concert to perform various completion and testing operations within the well bore. In one such completion structure, the pack-off assembly is located below the uppermost (primary) packer of the well when the pack-off work string is in the well. With relatively large well bores, this does not present a problem since the inner diameter of the packer is large enough to allow the pack-off assembly and work string to be withdrawn through the packer. However, with smaller well bores, it quickly becomes impractical to engineer the complexity of the pack-off assembly to a size small enough to be withdrawn through a packer with an inner diameter of five inches or less.
After completing the gravel pack, a reversing of the circulation in the pack-off work string removes any gravel-laden slurry from the work string. When the circulation is reversed, clean fluid is pumped down the path previously used for the returning proppant, and thus the remaining gravel-laden slurry is forced back up the path previously used to deliver the slurry to the packing zone.
An extremely important factor in achieving the gravel pack of the production zones of an oil or gas well is the ability to achieve seals throughout the system to prevent transmission of fluids to areas where they are neither required nor desired. With all of the moving parts associated with aligning gravel pack ports, supply streams, return flow streams, the gravel pack work string must be moved up and down in the well bore to numerous locations. During these movements, and certainly when the tool string is in place to accomplish a function, the various zones of the well must be isolated from one another.
One such system is described in U.S. Pat. No. 4,606,408 to Zunkel et al., commonly assigned to the present invention. Zunkel et al teaches gravel packing of multi-zone wells, however, the gravel pack assembly is located below the primary packer and essentially uses inverted cup packers to maintain zone integrity. Therefore, the complexity of the system and its location makes it impractical for well bores of less than five inches for reasons previously stated.
Therefore, what is needed in the art is an oil and gas well gravel packing system that permits packing of multiple production zones for use in well bores having packers with smaller inside diameters and a primary packer seal.